Thermosensitive data-carrier designed for the recording of information and a method of recording information on such a data-carrier

ABSTRACT

The present invention relates to data-carrier intended for the high-density recording of information by the use of an etching radiation beam. The data-carrier comprises a substrate upon which there are deposited a very thin film which is absorbent at the wavelength of the etching beam, and a film which is thermodegradable at a temperature lower than the degradation temperature of the absorbent film, the heat developed in the absorbent film being selectively diffused towards the adjacent thermodegradable film.

The present invention relates to data-carriers capable of being recordedby means of a radiation beam through conversion of the energy carried bythe beam into thermal energy.

Thermo-etching makes it possible to record data on data-carriers whichare other than photosensitive. This type of etching generally makes itpossible to produce data-carriers with very high resolution, theresolution being only limited by the size of the etching spot. Thedata-carriers obtained can be stored and exposed to the light withoutany danger, and may be read out again by a concentrated radiation beamwhose intensity is lower than the intensity of the beam used to etch thedata-carrier at the time of recording.

A first kind of material is used for the production of thermo-sensitivedata-carriers: these are metallic materials deposited in thin film formupon substrates, e.g. materials such as bismuth, cadmium or silver forexample. The sensitivity of these materials is generally low and theintensity of the etching beam and hence the power of the sourceproducing it, must be high. By way of example, the temperature to beachieved in order to bring about local degradation of a bismuth layer isaround 1500° C. and this requires the use of powerful, and thereforeexpensive, laser sources. On the other hand, films of this kind arehighly absorptive at the wavelengths used for the etching operation,since the diffusion length is of the order of 100 to 150 A units.

A second kind of materials can be used for the production ofthermo-sensitive data-carriers: these are organic materials which arethermodegradable at low temperature, such for example as nitrocelluloseor polymethyl-metacrylate (PMMA). Films of this kind have the advantagethat they degrade at low temperature (between 100° and 150° C.) but havelow absorption in the range of laser wavelengths used. To achieveadequate absorption of the radiation, it is necessary to deposit thesematerials in the form of a thick film of at least one micron, the depthof penetration for an absorption of up to at least 60% of the radiationbeing of the order of 1 micron. Moreover, the heat capacity of films ofthis kind is large and the sensitivity is therefore mediocre.

The invention relates to a thermo-sensitive data-carrier comprising asubstrate upon which there has been deposited a thermo-sensitive filmconstituted by a thin metal film and an organic film thermodegradable atlow temperature.

According to the invention, there is provided a thermo-sensitivedata-carrier for the recording of information by etching with aconcentrated recording radiation, comprising at least a thermo-sensitivefilm formed by a film thermodegradable at a temperature T₁ and anabsorbent film, in contact with the thermodegradable film, for absorbingsaid recording radiation, converting said radiation into heat andselectively diffusing said heat to said thermodegradable film, and saidthermodegradable film raising to its degradation temperature T₁.

The invention will be better understood and other of its featuresrendered apparent from a consideration of the ensuing description givenin relation to the attached drawings:

FIG. 1(a) illustrates the first embodiment of the thermo-sensitivedata-carrier in accordance with the invention, before recording;

FIG. 1(b) illustrates the same embodiment as in FIG. 1(a) afterrecording;

FIG. 2(a) illustrates a second embodiment of the thermo-sensitivedata-carrier in accordance with the invention, prior to recording;

FIG. 2(b) illustrates the same embodiment as in FIG. 2(a) afterrecording;

FIG. 3(a) illustrates a third embodiment of the data-carrier inaccordance with the invention prior to recording;

FIG. 3(b) illustrates the same embodiment as in FIG. 3(a) afterrecording;

FIG. 4 illustrates the method of manufacture of a die designed forpressing of data-carriers from a data-carrier in accordance with theinvention.

In FIG. 1(a), a thermo-sensitive data-carrier in accordance with theinvention has been shown. It comprises a substrate 1 having low thermaldiffusion, this substrate being formed for example from glass which istransparent to a radiation if, after etching of the data-carrier, it isdesired to read information across the substrate, or ofpolyethyleneterephtalate for example. It also comprises athermo-sensitive film constituted by a first film 2 of relatively smallthickness, in the order of one tenth of a micron, thermodegradable atlow temperature (between 100° and 150° C.) (this film may be constitutedfor example by nitrocellulose or PMMA) and a second film 3 of very smallthickness, of the order of one hundredth of a micron, constituted by ametal or semi-metal; this second film should be highly absorbent at thewavelength of the read-out radiation, this radiation coming from a lasertype source which is the only one capable, when the radiation itproduces is concentrated, of furnishing a spot of sufficiently smallsize to record microelements. The thickness of the metal film depositedis chosen in such a fashion that the major part of the incidentradiation is absorbed; a small thickness is only required since thedepth of penetration of the radiation is small, and at one hundredth ofa micron the film absorbs more than 60% of the radiation with such asmall thickness, the thermal energy created in the metal by the lightvibrations, is not diffused very widely at the surface and remainsconcentrated in the neighbourhood of the point of impact of theradiation; the calorific capacity per unit area, in order to raise thisfilm to a given temperature, is therefore low.

For etching, the data-carrier thus constituted is subjected to anincident etching radiation at the metal face 3. This etching radiationis almost completely absorbed by the metal film and the result is thatlocal heating of the film occurs, this heating being limited to theimmediate neighbourhood of the point of impact of the radiation.Temperatures of the order of 100° C. to 200° C. can be very rapidlyreached because of the high absorptivity of the metal and its lowcalorific capacity. This heat is transmitted towards the underlying zoneof the thermodegradable layer 2 which, as indicated earlier, is lowtemperature degradable, (in the neighbourhood of 150° C.). This film istherefore locally degraded; this degradation may take the form of amicroscopic explosion which mechanically attacks the metal film in thezone which has diffused the heat, or of a chemical decomposition, thedecomposition product chemically reacting with the metal film, againwithin the zone from which the heat diffused. This latter chemicaldegradation may be brought about the nitrous products, for examplenitrocellulose, which, after decomposition, will liberate gases such asNO₃, NO₂ or O₂ capable of combining with the metal. The metal film thenundergoes modification and the data carrier thus obtained is readable bydetection of the variations in amplitude of the radiation reflected bythe metal film. The thus etched data-carrier has been shown in FIG. 1(b)where are shown, in the film 2, zones such as 21 which have undergonethermal degradation by the heat transmitted to them from the zone 31 ofthe layer 3, where the etching radiation arrives, the zone 31 then beingattacked either mechanically or chemically following the degradation ofthe underlying zones 21. Because of the large calorific capacity of thefilm 2, the zones of this film which are not directly in contact withthe exposed zones of the metal film, undergo no degradation.

FIG. 2(a) illustrates the second embodiment of the data-carrier inaccordance with the invention. As before, the carrier comprises asubstrate 1 covered by a thermo-sensitive film, the latter film beingconstituted in the same way, by a first thermodegradable film 2 ofrelatively small thickness (of the order of one tenth of a micron) and asecond very thin metal film (of the order of one hundredth of a micron).However, by contrast with the first embodiment, the substrate 1 is nolonger in contact with the thermodegradable film 2 but instead with themetal film 3. For etching, the data carrier thus created is subjected toan incident etching radiation which strikes the organic material film 2.This film having a thickness in the order of a tenth of a micron (tentimes less than the depth of penetration of the radiation at anattenuation of 1/e), only a very small part of the radiation is absorbedby the organic film, this absorption being quite insufficient to heatthe film and raise it locally to its degradation temperature. A verylarge part of the incident radiation is therefore transmitted to themetal film which, because of its thickness and its absorptioncharacteristics, absorbs the major part of the radiation and heats uplocally and rapidly to reach temperatures of between 100° and 200° C.;this heat is diffused, not towards the substrate which is arranged tohave low heat transfer, but towards the thermodegradable film 2 whichfor that reason is raised to its degradation temperature by thermaldiffusion; the metal film remains intact because it is far from havingreached its degradation temperature. The film 2 is locally destroyed inthe zones 23 in contact with the heated metal zone such as 31, in themanner shown in FIG. 2(b).

FIG. 3(a) illustrates a third embodiment of the thermo-sensitivedata-carrier in accordance with the invention, etching of this kind ofdata-carrier being performed in the same way that was adopted for thedata carrier described in reference to FIG. 1.

This data-carrier comprises a film 4 of large thickness,thermodegradable at low temperature which performs the function of thesubstrate 1 and of the thermodegradable film 2 in the data-carrier shownin FIG. 1. On this thermodegradable film there is deposited a very thinfilm of metal or semimetal. The etching radiation strikes the film 3, islargely absorbed by said film and converted into heat which iscommunicated to the underlying zones of the film 4 so that these latterzones are raised to the degradation temperature. The degradation ofthese zones brings about the degradation of the adjacent zones of thefilm 3 by chemical or mechanical effect. The data-carrier obtained afteretching (FIG. 3(b) then comprises a film in which holes have been formedwhose succession along a track, constitutes the transcription of therecording information.

The thermo-sensitive data-carriers designed for information recording,which have been described hereinbefore, are such that they can be etchedby laser-type radiation of a low power compared with the power levels ofthose used hitherto for thermal etching. By way of example, in order toetch a data-carrier of disc form whose linear transfer speed oppositethe etching spot, is fifteen meters per second, a point of mean length0.7 microns can be etched by a laser beam having a power of less than 10milliwatts at the level of the film, this corresponding to an exposuretime of around 50 nanoseconds with a 10 milliwatts radiation. Powers ofthis level are of the order of magnitude of those obtained fromsemiconductor-type laser sources. This considerably reduces the cost ofthe etching operation in relation to that of the kind of similaroperation performed using gas lasers for example.

The power required at the level of the film for the thermal etching of abismuth film alone having a thickness of two hundredths of a microndeposited upon a glass substrate, is of the order of 40 milliwattswhilst in the case of a polyethyleneterephthalate substrate it is of theorder of 20 milliwatts.

In the same fashion, an organic film of 1 micron in thickness depositedupon a glass or polyethyleneteraphthalate substrate, requires a power inthe order of 20 milliwatts to etch it.

In contrast, a double film according to the invention made up of a filmof bismuth, two hundredths of a micron in thickness and an organic filmone tenth of a micron in thickness deposited upon a glass substrate,only requires an etching power in the order of 4 milliwatts and, if itis deposited on a substrate formed by polyethyleneterephthalate,requires an etching power of only around 2.5 milliwatts. Again ofbetween 5 and 10 is achieved in terms of the power required, in relationto conventional thermo-sensitive data-carriers above mentioned.

Within the context of the embodiments of the thermo-sensitivedata-carrier in accordance with the invention, as shown in FIGS. 1 and2, it is possible to adjust the thickness of the thermodegradable film 2so that it acts as an anti reflex film vis-a-vis the read-out beam whenthe data-carrier of FIG. 1 is read-out across the substrate and when thedata-carrier of FIG. 2 read-out across the thermodegradable film. Inother words, if e is the thickness of the film, λ the wavelength of theread-out radiation and n the refractive index of this film, the phasedifference between the radiation reflected on the one hand by the firstface of the film 2 reached by the radiation, and on the other hand bythe second, emergent face of the data-carrier, is 2.ne when e=λ/4n, sothat the emergent radiations will be in antiphase and will causeextinction at the output, the radiation remaining inside thedata-carrier.

The invention is not limited to the specific embodiments describedhereinbefore. In particular, in order to form the substrate, othermaterials may be used provided that they have low heat transfercoefficients and good mechanical strength. The thermo-sensitive filmitself, provided it is formed by a film which is thermodegradable at lowtemperature (it can be formed with nitrous organic products) and by ahighly absorbent film, is in accordance with the invention.

The thickness of the etched film (in the order of 0.1 micron) iscompatible with the read-out devices in which the phase differencebetween the radiation passing through an etched zone is chosen to bearound λ/2, where λ is the wavelength of the read-out radiation.

This kind of thermo-sensitive data-carrier has the advantage that it canbe recorded and read-out immediately since there is no developingoperation, and consequently real-time monitoring of etching can beperformed.

Moreover, a data-carrier of this kind, after recording, is directlycompatible with the manufacture of a die intended for reproduction ofthe impression by a pressing operation.

In other words, as FIG. 4 shows, a recorded data-carrier of the kindshown in FIG. 2(b) comprising a substrate 1 and a thermo-sensitive filmconstituted by an absorbent film 3 and a thermodegradable film 2 whichis partially degraded, is covered by a conductive film 5. This film maybe a silver or gold film, for example. In a general way, it is chosen toform an electrode and permit the deposition of a metal film 6 by aplating technique. This film may be constituted by nickel. The die thuscreated by the film 6 constitutes a "negative" of the information andcan be used to produce data-carriers by pressing operations.

What we claim is:
 1. A thermo-sensitive data-carrier for the recordingof information by etching with a concentrated recording radiation,comprising at least a thermo-sensitive film deposited upon a thermallyinsulating substrate, said thermo-sensitive film being formed by anabsorbent film in contact with the substrate and a film thermodegradableat a temperature T₁, affixed to the absorbent film, the etchingradiation incident upon the thermodegradable film passing through saidfilm and being absorbed to a small extent only, the major absorption ofit taking place in the absorbent film which converts said radiation intoheat and selectively diffuses said heat to said thermodegradable film.2. A thermo-sensitive data-carrier for the recording of information byetching with a concentrated recording radiation, comprising at least athermosensitive film deposited upon a thermally insulating substrate,said thermosensitive film being formed by a film thermodegradable at atemperature T₁ in contact with the substrate and an absorbent filmsticked to the thermodegradable film, the etching radiation beingincident upon the absorbent film which diffuses the heat towards theunderlying zones of the thermodegradable film and decomposes them,producing local variations in the structure of the absorbent film bymechanical attack, decomposition products of the thermodegradable filmproducing explosive mixtures.